Udûn (pron. [ˈuduːːn]) was the Sindarin name of Utumno, the fortress of Melkor in the far north of the World. It was the first and greatest of Melkor's citadels, delved in the earliest days, the home to hosts of demons and monsters. From Udûn, Melkor troubled the world for millennia. From there, he destroyed the Two Lamps of the Valar, so that the Powers left Almaren, their dwelling-place in Middle-earth, and removed into the West. At the time of the awakening of the Elves, though, the Valar attacked Udûn in full force, and destroyed it utterly, carrying its master back to Valinor as their prisoner.

The name Udûn is much less common as a name for Melkor's fortress than the equivalent Utumno. In The Lord of the Rings it appears just once, in Gandalf's quote given above. It survived, though, as a name for the northern valley of Mordor that lay behind the Morannon. No doubt Sauron chose this name in memory of his ancient master's greatest stronghold.